The modifications that have occurred in the primary structures of growth hormone and prolactin during evolution will be traced. This will be done by sequencing the tryptic peptides that are separated by peptide mapping and using homologies with hormones of known structures to order the tryptic peptide. Maximum structural information with a few milligrams of hormone can be gained in this manner. A correlation of structure and biological activities will be attempted. The alteration of biological activity of the hormone after deliberate and specific modification will be determined. Chemical and enzymatic alterations will be carried out. Structural and biological differences in minor forms of growth hormone and prolactin detected in both pituitary extracts and purified preparations will be determined. One of these forms has already been isolated and was found to cause a decreased tolerance to glucose in dogs in amounts where growth hormone was ineffective.